When silicon is subjected to high-rate anisotropic plasma etching using the maximum possible mask selectivity and the best possible profile control, for example in the manner referred to in German patent document no. 42 41 045, a stable passivation which can easily be removed again from the etching base of the structures to be etched while maintaining the mask selectivity, i.e., as a result of ion bombardment with a low energy input per unit of time, has to be applied to the side walls of the structures to be etched. At the same time, a high density of etching species has to be provided for removing the silicon from the etching base.
Fluorine radicals from fluorine-donating etching gases such as SF6, NF3, ClF3, BrF3, etc., which are broken up in a high-density plasma, are conventionally used as the etching species. As the passivation species, primary candidates are Teflon-forming monomers from passivation gases such as C4F8, C3F6 or other fluoro(hydro)carbons which may have a low ratio of fluorine to carbon, such as 2:1 or less, which can also be broken up in a high-density plasma.
The Teflon-forming monomers form a side wall protective film which prevents an etching attack on the side wall and gives rise to the desired anisotropy of the etching, while an ion bombardment directed primarily towards the etching base ensures that the Teflon-like protective films there may be broken down again so that the etching base remains substantially free of the protective film, while the fluorine radicals, acting as the etching species, etch the exposed silicon surfaces on the etching base.
If the attempt is made to supply a plasma having fluorine-donating etching gases and polymer-forming passivation gases at the same time and to break these up therein in order to generate a high density of etching species and passivation species at the same time, then a damaging mutual influence and undesirable recombination of both species is observed; this means that instead of polymer films being formed on the side walls and efficient etching being carried out at the etching base, the fluorine radicals and the polymerizable Teflon formers react to form saturated fluorine compounds which are largely inactive with respect to silicon.
To take account of this problem, U.S. Pat. No. 5,498,312 refers to using high plasma density in particular to counter the undesirable recombination reaction with a correspondingly higher production rate of both species by way of a high density of both these inherently incompatible species. However, this approach results, referring to the Si etching rate achievable per kilowatt of plasma output, in relatively inefficient processes and is problematic with regard to profile control and the reactor contamination with polymers which occurs at the same time and which is primarily caused by the required excess of passivation gas compared with the etching gas.
An alternative approach is referred to in U.S. Pat. No. 6,303,512, where etching and passivation gases which are more compatible with one another are used. Thus, in that case, SF6 or ClF3 is used as the fluorine-donating etching gas, while passivation is achieved using oxygen and silicon tetrafluoride by depositing a protective film similar to SiO2 on the sidewalls of the structures to be etched. Fluorine radicals do not react or recombine with oxygen radicals or SiF4, so etching and passivation gases may be used as a steady-state gas mixture without problems. However, it is disadvantageous here that side wall passivation is brought about by comparatively hard films similar to SiO2 which require a higher energy input through a directed ion bombardment in order to be capable of being broken through on the etching base, and this greatly reduces mask selectivity. The etching process according to U.S. Pat. No. 6,303,512 therefore has to operate while taking into account the limits in view of mask selectivity, which increases the risk of undesirable roughness in the etching base and so-called “grassing”. Passivation with the aid of SiO2 also has the disadvantage that inhomogeneities in the energy input to the etched substrate bring about much more pronounced disruptive effects than in the case of Teflon-like films.
The German patent document no. 42 41 045 is directed to solving the problem of the “unpeaceful coexistence” between Si-etching fluorine radicals and monomers forming Teflon-like films by separating the generation thereof in time, i.e., alternating the two processes. In this way, Teflon-like films formed during so-called “passivation cycles” are removed again during subsequent “etching cycles” which are in themselves isotropic and are re-deposited lower down in the trenches created, so that a localized protective action is produced, by the entrainment of the side wall film down into the trenches. The problem here is the limited plasma stability during gas change, where changes in impedance in the plasma may result in a mismatch of the high-frequency or microwave radiation supplied, which gives rise to reflected output until the plasma discharge begins (“flickering”). Moreover, this process sometimes involves individual cases in which it may not be possible to make the sidewalls of the trenches created completely smooth, which may be disadvantageous for example for optical applications using mirror surfaces.
An object of the exemplary embodiment and/or exemplary method of the present invention was to provide a method for the anisotropic high-rate etching of a semiconductor substrate such as silicon using a plasma and a device suitable for implementing the method, by which the disadvantages of German patent document no. 42 41 045 such as process instabilities or transient phenomena temporarily associated therewith may be overcome and sidewalls of the trenches created which are as smooth as possible without fluting in the walls may always be achieved.